When to Plant Iris Bulbs in Oregon?

Written by

William Golder


Dorian Goodwin

when to plant iris bulbs in oregon

Iris Tenax, or the Oregon Iris, is native to western Oregon and the southwest portion of the Cascade Mountain.

Thanks to the state’s mild temperature and ample moisture from rain, these bulbs thrive well and produce the most beautiful blossoms in the spring. By knowing when to plant iris bulbs in Oregon, you can ensure their health with minimal effort.

Therefore, plant them from August to September to make them grow successful and healthy.


Best Time to Plant Iris Bulbs in Oregon

1. Grow Irises From August to September.

grow-irises-from-august-to-septemberThe native iris in Oregon grows best in warm soil and mild temperatures with ample rainfall, as these conditions initiate proper root establishment.

The best thing about growing them in late summer through early fall is that this period will supply their requirements and help them grow well. But what month should we plant them exactly?

August to September is the ideal planting season because the soil is not covered yet with frost, which will help iris bulbs get established before the frigid winter comes and later on, encourage them to bloom by spring.

Aside from that, the nighttime temperature of the state’s autumn lies between 38°F and 56°F, which meets irises’ growing needs of 40-50°F.

But to make their early growth more promising, ensure to keep the soil moist by watering it once a week if it is not too rainy. However, avoid letting the bulbs sit in soggy conditions to protect them from rot.

Furthermore, it is essential to know that early summer is not a recommended time to plant irises because extreme heat can damage them. In the worst-case scenario, plant death is possible due to stress from too-high temperatures.

2. Plant Irises Six Weeks Before the First Frost.

Source: ufseeds

Oregon falls between the USDA zones 4 and 9, and each location has different climates and varying first frost dates. Since they thrive in mild conditions, it is best to grow iris bulbs before freezing weather arrives.

And just like all other states in the Pacific Northwest, Oregon’s ideal planting time for iris bulbs in the ground is at least six weeks before the first frost date.

For this reason, you should plant them in zone 5 on September 1st, since the region’s winter frost most likely starts on October 13th.

On the other hand, you can plant as early as August 10th if you live in zone 4, since you will experience your first expected frost on September 21st.

Some varieties you can plant here are Dutch, German, Siberian, and Japanese irises. They are adaptable to most of the state’s environment and share the same planting time frame.

Zone Number Last Frost Date First Frost Date
Zone 4 Apr. 24th-May 12th Sept. 21st-Oct. 7th
Zone 5 Apr. 7th-30th Oct. 13th-21st
Zone 6 Apr. 1st-21st  Oct. 17th-31st
Zone 7 Mar. 22nd-Apr. 3rd Oct. 29th-Nov. 15st
Zone 8 Mar. 13th-28th Nov. 7th-28th
Zone 9 Feb. 6th-28th Nov. 25th – Dec. 13th

Tips for Planting Iris Bulbs Successfully in Oregon


  • Plant in full sun, depending on the variety you choose. Grow them somewhere they can get sufficient sunlight (six hours daily) to encourage healthy and proper blooming.
  • Iris plants prefer slightly acidic soil (6.5 to 6.8) to meet their growing needs. Therefore, amend your soil by adding sulfur or coffee grounds to make it less alkaline and to promote fertility.
  • Whether you grow your bulbs in pots or directly on the ground, planting them in well-draining soil is necessary because irises are sensitive to having too much water.
  • Break the soil by tilling it, and add organic matter to promote fertility and help the ground have proper drainage.
  • Feed your iris plant with the proper fertilizer (6-10-10) once the blooming period comes. Avoid using products rich in nitrogen so that the plant doesn’t focus its energy on foliage growth.
  • Remove emerging weeds during the blooming season that could compete with your irises. And cut out foliage with leaf-spot disease to prevent them from spreading.
  • Trim irises’ leaves up to 6 inches long and cut out dead blossoms after their blooming time to avoid diseases and repel pests.
  • Divide iris plants to prevent them from overcrowding and to avoid halting their blooms. Do this every three to five years. 
    • Do this task around July through September.
    • Pull out the clumps, remove old rhizomes and keep the new ones, and transplant the viable rhizomes.

Related: Best time to plant tomatoes, strawberries and garlic in Oregon


The proper planting time is indeed crucial to achieve successful blooms over the spring because irises need mild temperatures to grow well.

Therefore, growing them from late summer to early autumn is the best way to establish ideal root growth and healthy development.

Remember to put them somewhere with full sun and in well-draining soil to meet their needs. Apply fertilizer at a suitable time, protect them from weeds, and practice dividing irises to avoid overcrowding.

Follow the guide and tips above on when to plant iris bulbs in Oregon, and they will surely grow successfully!

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